Internal combustion engines are energy transforming mechanism used by the vast majority of automotive vehicles and comprise basically two main parts: one of more engine heads and the engine block. On the base of the head (s) are located the combustion chambers (in Diesel engines, in general, the combustion chambers are located on the piston heads) and on the engine block are located the cylinders and the crankshaft assembly. Valves of an internal combustion engine are housed in the head (s) and are a device that aims at allowing or blocking the entry or exit of gases in the engine cylinders.
The engine converts the energy produced by combustion of the (fuel and air) mixture in the combustion chambers to mechanical energy capable of imparting movement to the wheels. Intake valves are those that control the entry of gaseous mixture into the engine cylinder, and escape valves are those that permit exit of the gases after the explosion.
By reason of the different stresses to which a valve is subjected, its constructive configuration is, in general, very similar. Thus, as can be seen in FIG. 1, a valve 1 is constituted by a disc-shaped head 10 that comprises a seat region 2 and a neck region 3 that acts as a transition region for the rod 4, wherein at the rod end opposite the head is located the valve 1 tip 5.
Each valve portion is subjected to different working conditions, thus being stressed in different manners. The seat region is subjected to wear by impact and friction, its function being that of sealing against the seat/insert present on the head. It should be noted that, besides the wear by impact upon closing the valve, there is also wear by friction (adhesion when the valve turns in contact with the insert). The head region, in turn, should be resistant to corrosion. The rod region should be resistant to the wear and have low friction. It should be noted that as little amount of lubricating fluid as possible should pass through the region between the rod and the guide, in order to prevent the lubricant from reaching the combustion chamber. Finally, the valve tip has to be resistant to wear, since it receives constant pressure on the part of the actuator that forces the valve to open and close.
In short, valves should exhibit resistance to three different types of stresses, namely: mechanical, thermal and chemical. With regard to mechanical resistance, valves should exhibit impact resistance in the seat region and in the tip region. Regarding resistance to wear, the main parts affected are the seat region, the rod and the valve tip. In turn, resistance to pressure should be a characteristic of the head front. Finally, the fatigue strength, it is necessary due to the constant alternation between the traction stress and the compression stress.
The need for heat resistance results from the combustion temperature, from the high temperatures of the escape gases and from the fatigue caused by the alternation between high and low temperatures.
The chemical resistance becomes necessary to prevent corrosion, which occurs easily in the corrosive environment of gases, moisture and working temperatures to which a valve is subjected.
Thus, with a view to overcome the more and more demanding working conditions to which valves are subjected, it is also common for valves to be monometallic of special alloys, bimetallic or provided with inserts.
Monometallic valves are constituted by a single material and are applied to moderate requirement parts. Thus, for intake valves, one usually employ martensitic steels, preferably of chrome-silicon alloys, because of their excellent mechanical properties. With regard to escape valves, one usually makes use of chrome-nickel-manganese alloy steels because of the excellent properties of resistance to corrosion and to high temperatures.
Bimetallic valves have applications in situations of greater requirement, in which a specific material is applied for each valve part. By way of example, one uses a martensitic steel is used on the rod, in order to guarantee high resistance to wear. With regard to the head, one uses austenitic steel or a nickel-based alloy to guarantee resistance to corrosion at high temperatures. Naturally, these valves have a higher cost due to their manufacture process, exhibiting limitations and, therefore, not being justifiable for many of the applications.
The valve tip usually receives, by means of welding, an austenitic-alloy tablet (in this case of a monometallic valve) and of low-alloy hard-tempered austenitic steel (in this case, of bimetallic valve).
At present, the need for higher thermal efficiency and specific power of engine, markedly due to the limits of emission of pollutants and consumption of fuel and lubricating oil, has led to an increase in thermal stresses of Otto or Diesel engines, among which are valves. This, in some more recent applications, the reduction in durability of these components has been considerable, calling for improvements.
Until now, among the most common solutions of coating valves of prior-art engines, one can cite nitriding, which exhibits a negative performance regarding fatigue resistance, for instance. Another example is the case of titanium valves, which are used for race-car engines, but ham a very high cost and low resistance to wear, for which reason their surface should be coated with a titanium nitride (TiN) or titanium oxide (TiO) in order to offset the low resistance to wear.
There are still a few additional solutions for engine valves that make use of commercially known alloys such as Nimonic or Nireva alloys, but the cost of these materials does not compensate for the properties offered for most situations.
Among the various prior-art solutions, document U.S. Pat. No. 4,811,701 discloses an intake valve for an internal combustion engine provided with a cerium oxide coating, applied by thermal spray with a view to prevent formation of carbon deposited on the valve.
Document U.S. Pat. No. 7,562,647 discloses an inlet valve having a coating resistant to high temperatures, as well as an internal combustion engine provided with such a valve. The valve is partly coated so as to guarantee resistance to corrosion, receiving also a head hardening treatment. The protective film disclosed deals generically with a curable resin, including at least one metallic or ceramic material and at least one organic or inorganic binding agent.
Document U.S. Pat. No. 5,441,235 discloses a titanium-nitride coated valve, as well as its production method. The titanium valve is basically nitride, but in reality one does deposit a TiN film, but rather only pressurized nitrogen reaction with titanium takes place on the valve surface. Since this is a diffusion process, high temperatures—from 700 to 880° C.—occur, for achieving an adequate formation of nitrides. This process is based on an arch established between cathode and the valve (anode), losing efficiency and perhaps causing non-conformity of the film by the tip effect (which accumulate load). Thus, as a drawback of this prior-art document, there is the non-suitability of the process for coating steels, due exactly to high temperature.
U.S. Pat. No. 7,225,782 too discloses a valve that receives a titanium nitride treatment, applied by physical vapor deposition (PVD) with a view to promote a protective oxide film on the valve surface. The document also comments on the possibility of using a film of DLC, chromine nitride or WC/C deposited by PVD. With regard to the WC/C coating, one mentions the formation of multiple layers of tungsten carbide alternating with amorphous carbon layers. However, the PVD method has exhibits a number of difficulties regarding a homogeneous deposition, which results in an undesired behavior with respect to resistance of the valve to wear. Another disadvantage of the PVD process is the difficulty in depositing the coating onto all faces of the piece, called anode, since the PVD process requires that the anode faces to be coated should be exposed frontally to the cathode, source of material of the PVD.
Multilayer films deposited by PVD often have high residual stresses, especially DLC films. Although they have good properties with regard to friction, DLC does not resist to high temperatures, let alone those to which the valve will be subjected. It should also be noted that it does now work well in corrosive environments (DLCs usually have “pinholes” through which corrosion finds way as far as the substrate). It should also be noted that, in the case of WC/C by PVD, it is necessary to evaporate the tungsten from powder or targets, which requires much energy, since one should reach about 2500° C.
Thus, the solutions that make use of PVD have disadvantage with respect to the present invention. Unlike the PVD process, in the present CVD process used in the present invention, since this is a chemical deposition, in which the coating materials are introduced into the deposition chamber in the form of a gas, all the piece surface that are not protected will receive the coating, since the gas in the environment will react with the piece surface.
Even though there are a number of attempts to minimize the wear to which valves are subjected, the prior-art solutions do not provide a valve of internal combustion engine that concomitantly manages to exhibit a superior behavior in all the durability requirements.
A few examples that they affect the durability of the prior-art valves can be seen in FIGS. 2 to 7. Thus, one of the phenomena that affect the durability of the prior-art valves most result from intergranular corrosion (ITG).
The phenomenon may be described as corrosion that begins in the contour of the grain. Due to exposure to high temperature, the alloy chrome migrates to the contour of the grain, that is, there is formation of a chrome precipitate in the bordering region of the grain. As a result, the loss of chrome as an alloy element, which is essential to corrosion, leads to dissolution of the grain borders and of the adjacent regions (see FIGS. 3, 4, 6 and 7).
The result of such an effect leads to fracture of the valve, as shown in FIG. 5, which shows a prior-art valve where a part of the head region has been lost due to a main fracture (substantially parallel to the perimeter—see FIG. 6), showing secondary fractures (substantially orthogonal to the perimeter—see FIG. 4) as well.
FIGS. 6 and 7 illustrate in detail, the intergranular corrosion 500 times magnified, as well as the removal of a few grains from the AA and BB sections of FIG. 5, respectively.
Another wear mechanism that usually takes place on prior-art valves is known as hot gas corrosion (HGC). FIGS. 8 and 9 shows the occurrence of such phenomenon in a valve rod, and there may be removal of material from the rod surface.
The corrosion caused by the hot gas to which the valves are subjected is generally a uniform mechanism of corrosion associated, in most cases, to hot gases to which the exhaustion valves are subjected. It is generally related to oxidation, but attack by molten salts such as sulfiding (sulfide salts formed by the fuel and lubricating fluid) may also occur. Usually, the prior art tries to control this process of corrosion steel valves by forming highly adherent non-porous chrome-oxide layer, wherein the HGC phenomenon begins when the layer loses its protective capability.
A third common phenomenon that attacks valves is shown in FIG. 10. In this case, a failure in the valve that prevents rotary movement thereof may cause a small opening that allows passage of gases from combustion. These gases, in turn, lead to corrosion in the valve-seat region, since they have high temperature and are corrosive.
Such occurrence prevents the correct sealing which a valve should provide. In some cases, localized fusion may take place, which accelerates the corrosion phenomenon until the valve fails. This takes place because the constant passage of hot gases raises drastically the temperature in one localized and concentrated region (see arrows in FIG. 10), and it is impossible for that valve to enable correct operation of the engine. It should also be noted that this phenomenon has special incidence when the valve has sealing problems.
In the face of the foregoing, nobody had so far developed a valve to which a coating is applied by the chemical vapor deposition (CVD) process, imparting to the resulting valve long durability and keeping the manufacture cost acceptable.